The purpose of the study was to compare the normal baseline responses in the heart rate, blood pressure, and skin blood flow of healthy volunteers to responses after one or more of the following physiologic challenges: administraton of phenylephrine, atropine, and/or propranolol, tilting with and without administration of the aforementioned drugs, Valsalva maneuver, and mental stress (i.e. rapid math calculations). This study terminated in August 1998.